1. Field of the Invention
The present invention relates to a wireless power transmission system for transferring energy by a non-contact method in order to charge a device or system for example with electric power or supply or collect electric power to/from such a device or system.
2. Description of the Related Art
Non-contact wireless energy transfer technologies have attracted increasing attention these days in order to increase the mobility and water resistance of various electronic devices as much as possible, realize a wireless wall-mounted TV monitor, and charge and supply power to a big object such as an electric car.
As a novel non-contact power transmission method that would replace the conventional one that uses electromagnetic induction, a resonant magnetic coupling method has been proposed in United States Patent Application Laid-Open Publication No. 2008/0278264-A1, for example (see FIGS. 6 and 11). According to such a method, resonant mode coupling between resonant antennas is used to realize longer range and higher efficiency power transmission than the conventional electromagnetic induction method. It is thought that particularly if a resonant magnetic field is used, influence on surrounding organisms would be reduced compared to a situation where a resonant electric field is used.
The resonant magnetic coupling method will be described in further detail. As disclosed in Paragraph #0033 of the description of United States Patent Application Laid-Open Publication No. 2008/0278264, the self-resonance phenomenon, which is unique to a loop coil with two open ends, may be used to form a resonator. Alternatively, as disclosed in Paragraph #0038 of that US patent application, the resonator may also be formed as a coil loaded with a capacitor.
On the other hand, Japanese Patent Application Laid-Open Publication No. 2008-104319 (see Paragraph #0006, in particular) discloses an example in which a multi-core wire such as a Litz wire or a braided wire is used to form the wiring portion of a power-transmitting or power-receiving coil in order to reduce the quantity of heat generated by a large amount of current that flows through the coil.
Furthermore, Japanese Patent Application Laid-Open Publication No. 7-263935 (see FIG. 3, in particular) proposes that multiple turns be connected in parallel with each other in order to increase the Q factor of an antenna (or coil) for use in non-contact power transmission without increasing its size too much.
In the conventional wireless power transmission system of the resonant magnetic coupling type, if the sizes of its power-transmitting resonator and power-receiving resonator are significantly different from each other, it is difficult to maintain high transmission efficiency. But to avoid such difficulty, the overall length of wiring used should be increased considerably.
It is therefore an object of the present invention to provide a wireless power transmission system that can maintain high transmission efficiency without wastefully increasing the overall length of wiring used even if the sizes of its power-transmitting and power-receiving resonators are significantly different from each other.